Method and apparatus for detection of ultrasound using a fiber-optic interferometer

Abstract

A method and apparatus for detection of ultrasonic vibrations using an interferometer built, in its preferred embodiment, with optical fibers is disclosed. The method and apparatus are particularly useful where perturbations and disturbances adversely affecting other interferometer configurations are encountered. Light from a laser is split into two orthogonal polarization modes which travel through different fiber-optic paths and are directed to the object being investigated. A waveplate and a polarization splitter are used to re-direct the reflected light into the optical path opposite from which it arrived. A second polarization splitter combines the returning light and an interference signal is obtained with photodetectors. The bias phase-shift between the interfering beams is fixed with a second waveplate. The detected signal is the result of motion which modulates the phase of the light reflected by the object being inspected.

What is claimed is: 
     
       1. An apparatus for detecting transient motion in an object comprising: a light source; a closed-circuit optical path comprising first and second arms of different optical-path-length that couple into orthogonal polarization modes of a third arm of said closed circuit optical path; means for splitting light from said light source into light propagating in opposite directions along said closed-circuit optical path; means for illuminating said object with light from said third arm and collecting and coupling back to said third arm reflected light from said object; means for directing said collected and coupled light traversing in opposite directions along said closed-circuit optical path such that it traverses the remaining portion of said closed-circuit optical path; means for controlling static phase difference between said collected and coupled light traveling in opposite directions along said closed-circuit optical path; means for producing an interference signal from said collected and coupled light traveling in opposite directions on said closed-circuit optical path; means for detecting said interference signal. 
     
     
       2. An apparatus as defined in claim 1 further comprising means for impeding said collected and coupled light from propagating a second time along the portion of said closed-circuit optical path first traversed by said collected and coupled light. 
     
     
       3. An apparatus as defined in claim 1 wherein said first and second arms of said closed-circuit optical path provide an optical-path difference equal in time to an odd multiple of half the period of the center frequency of sensed motion in said object. 
     
     
       4. An apparatus as defined in claim 1 wherein said closed-circuit optical path is determined by a non-polarizing beam-splitter, mirrors, a polarization splitter, a quarter-waveplate, and a focusing lens assembly. 
     
     
       5. An apparatus as defined in claim 1 wherein the said means for controlling static phase difference is comprised of a polarization controller located outside said closed-circuit optical path. 
     
     
       6. An apparatus as defined in claim 1 wherein said closed-circuit optical path is determined by a polarization-maintaining directional coupler, two polarization-maintaining fibers of different length comprising first and second arms of said closed-circuit optical path, a fiber polarization splitter, a third polarization-maintaining fiber comprising a third arm of said closed-circuit optical path, a polarization controller, and a focusing lens assembly. 
     
     
       7. An apparatus as defined in claim 1 wherein said closed-circuit optical path is determined by a polarization-maintaining directional coupler, two polarization maintaining fibers of different length comprising first and second arms of said closed-circuit optical path, two polarizing fibers of the same length, a second polarization-maintaining directional coupler, a length of polarization-maintaining fiber comprising a third arm of said closed-circuit optical path, a polarization controller, and a focusing lens assembly. 
     
     
       8. An apparatus as defined in claim 1 wherein the said closed-circuit optical path is determined by an optical circulator, a polarizing beam-splitter, two polarization-maintaining fibers of different length comprising first and second arms of said closed circuit optical path, a second polarizing beam splitter, a length of polarization-maintaining fiber comprising a third arm of said closed-circuit optical path, a polarization controller, and a focusing lens assembly. 
     
     
       9. An apparatus as defined in claim 1 wherein said means for producing an interference signal is comprised of a polarizing device aligned at 45 degrees with respect to the polarization axes of said closed-circuit optical path. 
     
     
       10. An apparatus as defined in claim 1 wherein said means for producing an interference signal is comprised of a polarizing beam-splitter aligned at 45 degrees with respect to the polarization axes of said closed-circuit optical path. 
     
     
       11. An apparatus as defined in claim 1 wherein said interference signal is proporational to the displacement amplitude of a narrow band ultrasonic signal present in said object. 
     
     
       12. A method of sensing transient motion in an object comprising the steps of: splitting a light source into light propagating in opposite directions on a closed-circuit optical path;   directing said light into a first arm and a second arm of different optical path length such that said arms couple said light into orthogonal polarization modes in a third arm;   directing said light from said third arm such that it illuminates said object being inspected,   collecting and coupling back into said third arm light reflected from said object resulting from illuminating said object with said light;   directing said collected and coupled light such that it traverses the remaining portion of said closed-circuit optical path;   controlling static phase difference between said collected and coupled light traversing in opposite directions along said closed-circuit optical path;   producing an interference signal from said collected and coupled light traversing in opposite directions along said closed-circuit optical path;   detecting said interference signal.   
     
     
       13. A method of sensing transient motion as defined by claim 12 wherein splitting said light source into light propagating in opposite directions along said closed-circuit optical path is accomplished with a polarization-maintaining directional coupler. 
     
     
       14. A method of sensing transient motion as defined by claim 12 wherein said first, second, and third arms of said closed-circuit optical path consist of polarization-maintaining optical fibers of different length and said coupling to orthogonal polarization modes of said third arm is accomplished with a polarizing beam-splitter. 
     
     
       15. A method of sensing transient motion as defined by claim 12 wherein illuminating said object and collecting and coupling back into said third arm the reflected light is accomplished by a lens assembly focused on said object. 
     
     
       16. A method of sensing transient motion as defined by claim 12 wherein directing said reflected light such that it traverses the closed-circuit optical path is accomplished by converting the output light from said third arm into circular polarized light using a polarization controller, and using a polarizing beam-splitter to redirect said circular polarized light into the arm opposite from that arm through which such light arrived at said beam-splitter. 
     
     
       17. A method of sensing transient motion as defined by claim 12 wherein controlling static phase difference between light traversing in opposite directions along said closed-circuit optical path is accomplished by a polarization controller located outside said closed-circuit optical path. 
     
     
       18. A method of sensing transient motion as defined by claim 12 wherein producing an interference signal from said collected and coupled light traversing in opposite directions along said closed-circuit optical path is accomplished by a polarizing beam splitter with its axes forming 45 degrees with respect to the polarization axes of the closed-circuit path. 
     
     
       19. A method of sensing transient motion as defined by claim 18 wherein detecting said interference signal is accomplished using a photodetector at each output port of said polarizing beam-splitter and subtracting signals detected by photodetectors. 
     
     
       20. A method of sensing motion as defined by claim 12 wherein said interference signal is proportional to the displacement amplitude of a narrow band ultrasonic signal present in said object.